1. Field of the Invention
The present invention relates to line transient protection circuits. In particular, the present invention relates to highly efficient line transient protection circuits for high power loads that are designed to operate through a high line transient (e.g. military and medical applications, etc.).
2. Background of the Invention
In general, transient electric phenomena occur, for instance, when a load is suddenly changed and an appreciable time elapses before the power level and circuit adapt to the new conditions. The voltages and currents during the intermediate time are known as transient. A transient may further be described as the momentary departure of a characteristic from steady-state conditions and back to steady-state conditions as a result of a system disturbance, such as a load or line change.
In high power load applications, line transient protection circuits (or voltage limiting circuits) have traditionally been designed using a power bipolar transistor. One well-known bipolar approach is the darlington power transistor transient protection circuit (1) is shown in FIG. 1 The darlington configuration typically includes a primary circuit having an input VIN (4), a bipolar transistor Q1A and Q1B (54) and an output VIN′ (5). A secondary leg connected to the primary leg between VIN and the bipolar transistor (54) includes a resistor R1 (34). The secondary leg is then fed into the bipolar transistor (54). A zener diode VR1 (28) is further arranged between the resistor R1 and the bipolar transistor (54) where the anode of VR1 is grounded (26). Additionally, the transient protected load depicted as RL circuit leg (50) is arranged between the bipolar transistor (54) and VIN′ where the return RL leg is grounded (26). However, a major disadvantage of the darlington circuit (1) is that it is not very efficient with respect to power consumption. In particular, at mid to high current levels, the power bipolar transistor suffers from poor current gain requiring high base current and high forward collector to emitter voltage drop, thus, is typically inefficient.
Another traditional approach to transient protection circuitry is to use a power MOSFET transient protection circuit as shown in FIG. 2. The power MOSFET transient protection circuit (2) typically includes a primary circuit having an input VIN (4), an n-channel enhancement power MOSFET Q1 (8) and an output VIN′ (5). A secondary leg connected to the primary leg between VIN and Q1 includes a resistor R1 (34). The secondary leg is then fed into the gate connection of Q1. A zener diode VR1 is further arranged between the resistor R1 and Q1 where the anode end of the zener diode is grounded (26). Additionally, a load, depicted as an RL circuit leg 50 is arranged between Q1 and VIN′ where the return of the RL leg is grounded (26). However, a major disadvantage of the power MOSFET transient protection circuit (2) is that it is typically more inefficient with respect to power consumption than that of the darlington transient protection circuit (1). Thus, one of the overall primary disadvantages of the aforementioned designs is that they are not power efficient.
It would be advantageous to provide a highly efficient line transient protection circuit for high power loads. In particular, it would be beneficial to provide a transient protection circuit that is adapted to operate through a high transient line. It would further be ideal to provide a transient protection circuit that can be used to protect medium to large current circuit from line transients.